1. When the user moves the cursor up, show him a small popup with a text "you are moving the cursor up". In my oppinon the popup can be 50% the screen, but some users may think is it too big.

2. When the user selects a text like "ca", show him an image with a cat. That will be grait! All the people likes cats.

3. When the user tries to close the browser, close the browser, and after opera finishes to close, reopen the browser and tell the user using an alert box: "You have closed the browser". It may appear a loop here, but you may also set a limit. Let's say 25 times.

4. When the user uses CTRL + F to search in the page, open something. I don't know what. Think something. You can show a door opening or whatever. That will be funny!

5. The last one. When the user tries to search something in the address bar, give him some suggestions and preddictions. It is important not to make a setting in the browser for this, so the use can't disable. It has no sense. Let him see those suggestions, even if there are unnecesarry..

Have a nice day!